Separated readout geometry calculation from geometry build in MuonGeoModel::Mdt

This is groundwork for a fix for the SQLite database scenario which will come after this MR is found to be safe.

In the present system, a class called MuonGeoModel::Mtd builds geometry. In the new, SQLite-based detector description system, the geometry is restored rather than built. As it turns out, the "build" method of MuonGeoModel::Mdt has a few side-effects; it fills a data structure describing cutouts. This functionality is still needed for the new DD. So, I have separated it out so that the data structure can be filled without creating redundant geometry.